This invention relates to optical filters. More particularly, it is concerned with infrared light transmissive, visible light absorptive optical filters.
Selective optical absorption filters are used in a variety of applications including video displays, analytical instrumentation, photographic equipment, and light sources, both for color correction and for heat dissipation. The term "selective optical absorption filter" as used herein is defined as a medium which substantially absorbs certain selected wavelengths of light while substantially transmitting others. The term "light" is intended to include electromagnetic radiation in the near ultraviolet region of the spectrum, the visible region, and the near infrared region. Somewhat arbitrarily, the near-ultraviolet region of the electromagnetic spectrum may be considered to cover wavelengths from about 0.3 .mu.m to 0.4 .mu.m, the visible region to cover from 0.4 .mu.m to 0.7 .mu.m, and the near infrared region to cover from 0.7 .mu.m to about 5.0 .mu.m.
High temperature light sources such as incandescent lamp filaments, carbon arcs, high intensity discharge lamps, etc., radiate a substantial portion of their energy output in the near-infrared region of the electromagnetic spectrum. Because of the heating effects of near-infrared radiation, it is desirable in certain applications such as photographic projector lamps and the like to dissipate these longer non-visible wavelengths of light to protect the heat-sensitive photographic medium in the projector from damage.
Selective optical absorption filters have been employed in a variety of devices to dissipate the infra-red heat generating wavelengths of light from high temperature light sources. In one arrangement, a selective optical filter which transmits visible wavelengths and absorbs infrared wavelengths is placed between the light source and the object to be illuminated. In yet another arrangement, a reflective selective optical absorption filter is placed behind the light source, frequently forming a part of the lamp housing. The reflective filter transmits unwanted infrared wavelengths of light out the back portion of the lamp housing while reflecting the desired visible wavelengths of light forward through the front portion of the lamp housing.
Selective optical absorption filters have been fabricated in the prior art generally as dye filters or as quarter wave filters. Dye filters comprise a material which absorbs the unwanted light wavelengths dispersed in a medium which is transparent to the desired wavelengths. Frequently, dye filters do not possess the necessary thermal stability for applications involving heat dissipation in high temperature light sources.
Quarter wave filters comprise one or more films of a thickness equal to a quarter of the wavelength, or some small integral multiple thereof, of the light to be removed from the light stream. If a wide band of wavelengths is to be filtered, a number of quarter wave films must be deposited on the filter substrate.